Ever since the introduction of PTFE to the market many years ago, fabricators have encountered problems with joining the material to itself or to other materials. These problems have been even more evident when attempting to join sintered PTFE elements. During the ensuing years, a number of bonding and/or welding processes have been developed. For example, U.S. Pat. No. 3,207,644 to Hobson et al. describes a welding process whereby pure PTFE elements are joined by subjecting them to heat and pressure followed by water quench. U.S. Pat. No. 4,701,291 to Wissman discloses a complex process for welding pure PTFE components in a mold and utilizing a bath of molten metal or salt. Neither of these references makes any representation that its process is applicable to filled sintered PTFE components and neither shows any recognition that filled sintered PTFE material presents different processing problems than pure PTFE.
Other bonding or welding processes call for enhancing the integrity of joints by the use of an intermediate layer of fluorine-containing resin such as fluorinated ethylene propylene (FEP) or perfluoroalkoxy (PFA) resin. Examples of such methods are described in U.S. Pat. Nos. 4,211,594 to Freitag et al., 4,073,856 to Chu and 2,833,686 to Sandt. Other prior art patents describe processes for fusing unsintered PTFE surfaces. Among this group are U.S. Pat. Nos. 3,645,820 to Clary, 4,283,448 to Bowman and 4,364.884 to Traut.
These and other methods have been useful for joining homogeneous PTFE, but experience reveals that such methods are not satisfactory for joining filled sintered PTFE components. Sometimes tensile strength across the bond is lower than that of the main body of material, sometimes the surfaces in the area of the weld are poor and sometimes, chemical and/or temperature resistance is reduced. For example, a silica filled PTFE material welded with an FEP intermediate layer was found to have a tensile strength across the weld of about 1350 psi whereas the silica filled PTFE material itself has an average tensile strength of about 2000 psi or more. In another trial, the method of U.S. Pat. No. 3,207,644 was employed with a silica filled PTFE and the tensile strength across the weld was about 1300 psi and elongation was severely reduced. For reason or reasons as yet unknown, the presence of filler(s) in PTFE materials inhibits the obtaining of satisfactory welds.